There should be drinking fountains at school he can use. You could try to ask the school nurse (or his teacher) if you can leave a case of water in her office every week. You might need to get a letter from your doctor explaining there is a medical necessity. Then, he wouldn't have to carry them around (extra weight in his backpack), and they would already be at school.
I think the camel back suggested is the greatest, most convenient idea in this case. He'll have it with him at all times, without even having to think about it. I don't think the other kids will make fun of him or anything like that. It's so hard in elementary school. Who knows, maybe he'll start a trend and the other kids will want one too.